Compatibility of Internal Transport Barrier with Steady-state Operation in the High Bootstrap Fraction Regime on DIII-D. PDF Download
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Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Recent EAST/DIII-D joint experiments on the high poloidal beta tokamak regime in DIII-D have demonstrated fully noninductive operation with an internal transport barrier (ITB) at large minor radius, at normalized fusion performance increased by e"0% relative to earlier work. The advancement was enabled by improved understanding of the "relaxation oscillations", previously attributed to repetitive ITB collapses, and of the fast ion behavior in this regime. It was found that the "relaxation oscillations" are coupled core-edge modes 2 amenable to wall-stabilization, and that fast ion losses which previously dictated a large plasma-wall separation to avoid wall over-heating, can be reduced to classical levels with sufficient plasma density. By using optimized waveforms of the plasma-wall separation and plasma density, fully noninductive plasmas have been sustained for long durations with excellent energy confinement quality, bootstrap fraction e"80%, [beta]N d"4, [beta]P e"3, and [beta]T e"2%. Finally, these results bolster the applicability of the high poloidal beta tokamak regime toward the realization of a steady-state fusion reactor.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Recent EAST/DIII-D joint experiments on the high poloidal beta tokamak regime in DIII-D have demonstrated fully noninductive operation with an internal transport barrier (ITB) at large minor radius, at normalized fusion performance increased by e"0% relative to earlier work. The advancement was enabled by improved understanding of the "relaxation oscillations", previously attributed to repetitive ITB collapses, and of the fast ion behavior in this regime. It was found that the "relaxation oscillations" are coupled core-edge modes 2 amenable to wall-stabilization, and that fast ion losses which previously dictated a large plasma-wall separation to avoid wall over-heating, can be reduced to classical levels with sufficient plasma density. By using optimized waveforms of the plasma-wall separation and plasma density, fully noninductive plasmas have been sustained for long durations with excellent energy confinement quality, bootstrap fraction e"80%, [beta]N d"4, [beta]P e"3, and [beta]T e"2%. Finally, these results bolster the applicability of the high poloidal beta tokamak regime toward the realization of a steady-state fusion reactor.
Author: Publisher: ISBN: Category : Languages : en Pages : 14
Book Description
Conclusions of this report are: (1) In our scans of q{sub min} and q95, the bootstrap current fraction increased with q95 but did not continue to increase with q{sub min} above about 1.5 as expected by f{sub BS} H"q[beta]{sub N}; (2) With existing control tools, q{sub min} H"1.5 appears optimal for maximizing bootstrap current if the calculated ideal wall limit can be reached (only narrowly more so than q{sub min} H"1.1); (3) q{sub min} H"2 discharges achieved lower [beta]{sub N} and calculated n = 1 [beta]{sub N} limits, had increased transport, lower density, lower temperature gradients, and as a result did not produce as much bootstrap current; (4) Highest f{sub BS} achieved at highest q95 (=6.8), but scan suggests lower q95 is required for more reactor relevant fusion gain G H"[beta]{sub N}H9/q952; (5) New tools (off-axis NBI, more ECCD) may allow access to higher [beta]{sub N} limits and higher bootstrap fractions.
Author: Publisher: ISBN: Category : Languages : en Pages : 7
Book Description
Discharges characteristic of the quiescent double barrier (QDB) regime [1] are attractive for development of advanced tokamak (AT) scenarios relevant to fusion reactors [2] and they offer near term advantages for exploring and developing control techniques. We continue to explore the QDB regime in DIII-D to improve understanding of formation and control of these discharges and to explore scaling to steady-state reactors. The formation of an internal transport barrier (ITB) provides a naturally peaked core pressure profile. This peaking in density in combination with the H-mode-like edge barrier and pedestal provide a path to high performance. We have achieved [beta]{sub N}H{sub 89P} H"7 for several energy confinement times (d"25 [tau]{sub E}). We discuss here a combination of modeling and experiments using electron cyclotron heating (ECH) and current drive (ECCD) to demonstrate steady state, current-driven equilibria and control of the current distribution, safety factor q, and density profile. Experimental conditions leading to formation of the QDB discharge require establishing two distinct and separated barrier regions, a core region near [rho] H"0.5 and an edge barrier outside [rho]> 0.95, [rho] is the square root of toroidal flux (radial coordinate). A region of higher transport due to a change in polarity of the E x B shearing rate [1] separates the core barrier from the H-mode edge. It is this separation in barriers that so far has required use of counter-NBI to establish QDB conditions. Balanced NBI should also allow this separation of barriers. The edge corresponds to the quiescent H-mode (QH) conditions [3]. In this quiescent edge region, the normally observed transient loss associated with edge-localized-mode (ELM) activity is replaced with a steady particle loss driven by a coherent oscillation residing outside the pedestal region. This edge harmonic oscillation (EHO) [2] typically exhibits 2 or 3 harmonics of a fundamental frequency near 10 kHz. We find this combination of a core ITB and the QH-mode edge to be extremely robust and to produce slowly varying, high performance discharge parameters, Fig. 1, for long durations H"3 s. These conditions are generally limited by the duration of the NBI system and a slow evolution to lower q values as the Ohmic current moves inward on the resistive time scale for diffusion.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
OAK-B135 The authors have initiated an experimental program to address some of the questions associated with operation of a tokamak with high bootstrap current fraction under high performance conditions, without assistance from a transformer. In these discharges they have maintained stationary (or slowly improving) conditions for> 2.2 s at[beta][sub N][approx][beta][sub p][approx] 2.8. Significant current overdrive, with dI/dt> 50 kA/s and zero or negative voltage, is sustained for over 0.7 s. The overdrive condition is usually ended with the appearance of MHD activity, which alters the profiles and reduces the bootstrap current. Characteristically these plasmas have 65%-80% bootstrap current, 25%-30% NBCD, and 5%-10% ECCD. Fully noninductive operation is essential for steady-state tokamaks. For efficient operation, the bootstrap current fraction must be close to 100%, allowing for a small additional ([approx] 10%) external current drive capability to be used for control. In such plasmas the current and pressure profiles are rightly coupled because J(r) is entirely determined by p(r) (or more accurately by the kinetic profiles). The pressure gradient in turn is determined by transport coefficients which depend on the poloidal field profile.
Author: Publisher: ISBN: Category : Languages : en Pages : 5
Book Description
OAK-B135 Recent results from DIII-D address critical internal transport barrier (ITB) research issues relating to sustainability, impurity accumulation and ITB control, and have also demonstrated successful application of general profile control tools. In addition, substantial progress has been made in understanding the physics of the Quiescent Double Barrier (QDB) regime, increasing the demonstrating operating space for the regime and improving performance. Highlights include: (1) a clear demonstration of q-profile modification using electron cyclotron current drive (ECCD); (2) successful use of localized profile control using electron cyclotron heating (ECH) or ECCD to reduce central high-Z impurity accumulation associated with density peaking; (3) theory-based modeling codes are now being used to design experiments; (4) the operating space for Quiescent H-mode (QH-mode) has been substantially broadened, in particular higher density operation has been achieved; (5) absolute ([beta] 3.8%, neutron rate S{sub n} d"5.5 x 1015 s−1) and relative ([beta]{sub N}H9 = 7 for 10 [tau]{sub E}) performance has been increased; (6) with regard to sustainment, QDB plasmas have been run for 3.8 s or 26 [tau]{sub E}. These results emphasize that it is possible to produce sustained high quality H-mode performance with an edge localized mode (ELM)-free edge, directly addressing a major issue in fusion research, of how to ameliorate or eliminate ELM induced pulsed divertor particle and heat loads.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
We continue to explore Quiescent Double Barrier (QDB) operation on DIII-D to address issues of critical importance to internal transport barrier (ITB) plasmas. QDB plasmas exhibit both a core transport barrier and a quiescent, H-mode edge barrier. Both experiments and modeling of these plasmas are leading to an increased understanding of this regime and it's potential advantages for advanced-tokamak (AT) burning-plasma operation. These near steady plasma conditions have been maintained on DIII-D for up to 4s, times greater than 35[tau][sub E], and exhibit high performance with [beta][sub N]> 2.5 and neutron production rates S[sub n] [approx] 1 x 10[sup 16]s[sup -1]. Recent experiments have been directed at exploring both the current profile modification effects of electron cyclotron current drive (ECCD) and electron cyclotron (ECH) heating-induced changes in temperature, density and impurity profiles. We use model-based analysis to determine the effects of both heating and current drive on the q-profile in these QDB plasmas. Experiments based on predictive modeling achieved a significant modification to the q-profile evolution [1] resulting from the non-inductive current drive effects due to direct ECCD and changes in the bootstrap and neutral beam current drive components. We observe that the injection of EC power inside the barrier region changes the density peaking from n[sub e]/n[sub e] = 2.1 to 1.5 accompanied by a significant reduction in the core carbon and high-Z impurities, nickel and copper.
Author: M. MURAKAMI Publisher: ISBN: Category : Languages : en Pages : 7
Book Description
OAK-B135 The authors have initiated an experimental program to address some of the questions associated with operation of a tokamak with high bootstrap current fraction under high performance conditions, without assistance from a transformer. In these discharges they have maintained stationary (or slowly improving) conditions for> 2.2 s at {beta}{sub N} {approx} {beta}{sub p} {approx} 2.8. Significant current overdrive, with dI/dt> 50 kA/s and zero or negative voltage, is sustained for over 0.7 s. The overdrive condition is usually ended with the appearance of MHD activity, which alters the profiles and reduces the bootstrap current. Characteristically these plasmas have 65%-80% bootstrap current, 25%-30% NBCD, and 5%-10% ECCD. Fully noninductive operation is essential for steady-state tokamaks. For efficient operation, the bootstrap current fraction must be close to 100%, allowing for a small additional ({approx} 10%) external current drive capability to be used for control. In such plasmas the current and pressure profiles are rightly coupled because J(r) is entirely determined by p(r) (or more accurately by the kinetic profiles). The pressure gradient in turn is determined by transport coefficients which depend on the poloidal field profile.
Author: T. H. Osborne Publisher: ISBN: Category : Languages : en Pages :
Book Description
We continue to explore Quiescent Double Barrier (QDB) operation on DIII-D to address issues of critical importance to internal transport barrier (ITB) plasmas. QDB plasmas exhibit both a core transport barrier and a quiescent, H-mode edge barrier. Both experiments and modeling of these plasmas are leading to an increased understanding of this regime and it's potential advantages for advanced-tokamak (AT) burning-plasma operation. These near steady plasma conditions have been maintained on DIII-D for up to 4s, times greater than 35{tau}{sub E}, and exhibit high performance with {beta}{sub N}> 2.5 and neutron production rates S{sub n} {approx} 1 x 10{sup 16}s{sup -1}. Recent experiments have been directed at exploring both the current profile modification effects of electron cyclotron current drive (ECCD) and electron cyclotron (ECH) heating-induced changes in temperature, density and impurity profiles. We use model-based analysis to determine the effects of both heating and current drive on the q-profile in these QDB plasmas. Experiments based on predictive modeling achieved a significant modification to the q-profile evolution [1] resulting from the non-inductive current drive effects due to direct ECCD and changes in the bootstrap and neutral beam current drive components. We observe that the injection of EC power inside the barrier region changes the density peaking from n{sub e}/n{sub e} = 2.1 to 1.5 accompanied by a significant reduction in the core carbon and high-Z impurities, nickel and copper.
Author: Publisher: ISBN: Category : Languages : en Pages : 5
Book Description
Neutral beam heated discharges with internal transport barriers (ITB) have been observed in most of the world's major tokamaks including DIII-D, TFTR, JT-60U and JET. Improved core confinement has been observed over a range of q profiles, including negative central magnetic shear (NCS) and weak positive shear. In discharges with an L-mode edge, the duration of ITBs is generally limited to a few energy confinement times ([tau]{sub e}) by low-n MHD activity driven by the steep core pressure gradient or by q{sub min} passing through low-order rational values. In order for ITBs to be useful in achieving a more compact advanced tokamak, the radius of the ITB must be expanded and the pressure gradient must be controlled in order to optimize [beta]{sub N} and the bootstrap alignment. In this paper, the authors discuss the results of recent experiments on DIII-D to produce and sustain ITBs for longer pulse lengths. Three techniques are evaluated: (1) reduction of neutral beam power (P{sub NBI}) and plasma current to form a weaker ITB which can be sustained; (2) use of an ELMing H-mode edge to help broaden the pressure profile and improve MHD instability; (3) modification of the plasma shape (squareness) to achieve smaller ELMs with a lower density pedestal at the edge.
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Author: M. MURAKAMI
Author: T. H. Osborne
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